The invention relates to a novel process for increasing the proportion of one epimer in epimer mixtures of certain pregna-1,4-diene-3,20-dione 16,17-acetal 21-esters.
DE-A 41 29 535 discloses epimeric pregna-1,4-diene-3,20-dione 16,17-acetal 21-esters with an antiinflammatory action. These have a butyl, isopropyl, sec-butyl, cyclohexyl or phenyl radical on the cyclic acetal ring, and their C-21 hydroxyl group is acylated by an acetyl or isobutyryl radical. The application describes how the respective R-epimer is obtained, starting from an R/S mixture, by preparative high-pressure liquid chromatography (HPLC). International Patent Application WO95/24416 describes a process for the epimer enrichment of pregna-1,4-diene-3,20-dione 16,17-acetal derivatives by silylation, fractional crystallization and acid hydrolysis.